1. Field of the Invention
The present invention relates to a pushbutton switch which has excellent electrical characteristics and which is convenient to use at night or in dark places when used as a terminal switch in electronic notebooks or in various portable communication devices such as cellular phones, car phones and the like.
Further, the present invention relates to a pushbutton switch which makes it possible for the wiring and the like inside the terminal device to be sufficiently protected against electromagnetic interference (EMI) from the outside as well as making it possible to prevent the occurrence of electrostatic discharge (ESD) in the printed wiring and the like due to the buildup of a high voltage electrostatic charge.
2. Description of the Prior Art
In one example of a prior art pushbutton switch, a printed-wiring board, which is equipped with a power source such as a light-emitting diode or the like, is provided with a keypad constructed from a flat non-working portion made from translucent silicone rubber, a bulging thin-walled skirt portion provided above the non-working portion, and a working portion supported by an upper part of the skirt portion. In this construction, the upper surface of the keypad, excluding the working portion there of, is covered by a white-colored film made from synthetic resin, with the upper surface of such synthetic resin film being formed into a black-colored printed membrane.
Further, in a dome style switch, a dome portion which rises in the shape of a dome is formed from metal, with the periphery of the dome portion being held in a fixed state by an adhesive film for fixing the dome portion. In this construction, when the dome portion is lowered, the upper electrode film positioned directly below the dome portion comes into contact with the lower electrode provided on the printed-wiring board.
Now, when the pushbutton switch according to the first example described above is used as an illuminated switch, the light emitted from the light source passes through the upper surface of the working portion to illuminate the numbers, letters, characters and other such symbols displayed on the working portion without any of such light leaking to the outside through the respective sides of the non-working portion, skirt portion and working portion. At this time, even though the surface of the white-colored synthetic resin film reflects the light emitted by the light source, a portion of such emitted light will be absorbed into the inside of the film, resulting in an insufficient reflectance. Accordingly, it becomes necessary for the number of light sources to be the same or nearly the same as the number of keys, and because the light-emitting d odes and the like that make up such light sources are expensive, the overall cost of the pushbutton switch becomes quite high.
Furthermore, when the keys of such prior art pushbutton switch are pressed with a finger, the presence of a high voltage electrostatic charge, for example, on the finger of an operator can cause an electrostatic discharge (ESD) in the printed-wiring board and the like.
Moreover, the effect of outside electromagnetic interference (EMI) on the wiring within the terminal device can give rise to electrical disturbances within the circuit and can cause the electrical characteristics of the device to deteriorate. In this regard, the prior art pushbutton switch described above is not sufficiently shielded against such electromagnetic interference.
Further, in the prior art dome switch described above because only the dome portion is made of metal and the dome portion supporting member is made from an insulating synthetic resin film, the effect of outside electromagnetic interference (EMI) on the wiring and the like inside the terminal device can result in an adverse effect on the electrical characteristics of the device, and it is also possible for an electrostatic discharge to occur in the printed wiring and the like due to a buildup of a high voltage electrostatic charge.